The objectives of the research project are to develop the non-invasive technique of laser Doppler velocimetry as a means to measure blood flow in individual human retinal vessels, and to apply the technique to study the physiology of the human retinal circulation, to investigate the role played by alterations in retinal blood flow in the causation and development of ocular diseases, and to study the effects of systemic diseases on the retinal blood flow. To carry out these objectives, instruments capable of continuous absolute measurements of the velocity of red blood cells flowing in retinal vessels are currently being tested. These instruments will be applied to determine the absolute blood flow rate in the retinal vessels of a population of normal volunteers as well as a population of patients with a variety of systemic and ocular diseases. The variations of instantaneous red blood cell velocities during the cardiac cycle in these populations will also be determined. In addition, experiments on the effect of various controlled physiological conditions on retinal blood flow will be conducted.